Wireless Microphones

There are obvious advantages to using wireless microphone systems, especially if you move around a lot as a performer or if you're an actor in a drama where the shot is very wide and the boom microphone can't reach you without appearing in shot, but naturally, there are also some trade-offs.

Wireless microphone prices have dropped dramatically in recent years and this has resulted in their greater use in the Live music and Conference markets, also for amateur musicians and even in pubs for use as Karaoke microphones.

Depending on where you are in the world they are also known as Cordless microphones or Radio microphones.

Pro's and Con's of Wireless Microphones

The obvious benefit of using a wireless microphone (radio microphone / cordless microphone) is complete freedom of movement and no possibility of getting tangled up in your own microphone cable. It's not just microphones that benefit from wireless technology, guitarists, and even keyboard players can use wireless transmitters to feed their signal to the P.A. system.

The potential downsides of wireless microphones are radio interference from other transmitters e.g. taxis and other wireless microphones operating on the same frequency or even a nearby frequency, loss of signal due to too great a distance between the transmitter and receiver or phase cancellation of the radio wave as it bounces off different surfaces, then of course there's power failure - use fresh batteries in the transmitter before each important event.

Wireless microphone transmitters use a compressor circuit to reduce the large dynamic range of the microphone and then an expander circuit in the receiver to de-compress the signal. This 'compansion' technology is very good nowadays but it can still have some 'pumping' side effects on some sound sources. So where absolute fidelity is required it's best to use a wired microphone.

A dynamic microphone can be used with a wireless system and so can an electret microphone (also known as a tie clip microphone or personal microphone) a small voltage is required and this is available on modern transmitters but check first, don't just assume it will work, while a condenser microphone needs a power supply between the microphone and the transmitter.

The microphone connectors are usually proprietory so you will have to either buy the lavalier microphones or XLR leads already wired with the correct connector or buy adaptors.

Types of wireless microphone

There are two types of radio microphone, handheld and body pack (belt) transmitters.

Handheld microphones incorporate the transmitter into the body of the microphone and the antenna is situated where the lead would normally go. Controls can be situated externally but are usually inside the body where they wont accidentally get changed.

Another handheld option is a self contained transmitter with an XLR socket incorporated that plugs directly into the microphone, some will only work with Dynamic mic's others have an integral phantom power supply that allows you to use condenser mic's.

Body pack transmitters are typically the size and shape of a pack of cards and usually have a lavalier microphone or headset microphone plugged into them. You can also plug a Dynamic microphone lead with the appropriate connector into it. The antenna and microphone input are usually on the top of the pack along with the various controls such as audio gain and frequency selection.

An additional wireless transmitter and receiver system can be used to play back a backing track, fold-back from the rest of the band or 'talkback' from a control room to the headphone part of a headset microphone or in-ear-monitoring (IEM) moulded earpiece. Some headsets only have a microphone attached, they are used to get the microphone very close to the user so that the possibility of acoustic feedback is reduced.

The first wireless microphones employed the VHF (very high frequency) radio band and these are still typically used in the cheaper and amateur systems, they will provide a good quality output, however the latest ones use the UHF band (ultra high frequency) because they provide a wider frequency response and are less prone to radio interference in the UK (and no doubt other countries) as the VHF band becomes more congested.

How they work

The audio signal is modulated by a radio signal at a set frequency and de-modulated at the receiver (tuned to that same frequency) into an audio signal that is sent to the audio mixer or directly to the amplifier. Each wireless microphone kit has to be set to a different frequency from all the others used in that venue.

The first wireless microphone receivers had one antenna (Non-diversity) but the latest receivers employ a 'Diversity' system whereby there are two separate identically tuned receiver circuits each with their own antenna, then a 'signal strength' circuit analyses which of the receivers has the strongest signal and noiselessly switches between them accordingly many times a second. This avoids audio dropouts and any 'shash' where reception is poor. The antennae are positioned at right angles to one another to ensure that one receiver's antenna is always optimally aligned to the transmitter's antenna.

In the UK the cheaper wireless microphones tend to use de-regulated (free) frequencies and only a few switchable frequencies per kit, this is why for professional use there are more frequencies per kit and they are in 'paid for' frequency bands that guarantee a greater chance of protection from interference. Each country will have its own system of allocating frequencies and any payments due.

Some receivers have a 'squelch' control that can be altered, this is used to prevent a loud 'shash' being heard when the radio signal disappears, its threshold should be set to where it mutes the audio when the signal is poor but not so high that it keeps muting the audio when the signal is low but still usable. To set it up you will need an assistant to take the transmitter far enough away from the receiver so that the signal is almost useless and then set the threshold of the squelch to mute the output.

Some systems have this control pre-set internally and are not accessible to the user and some systems have a very high frequency control signal that has to be detected by the receiver before it will un-mute the audio.

There are digital wireless microphones available which make better use of the radio bands (more channels can be used than in the analogue domain) but care should be taken where conventional microphones are used as well since there is a processing delay as the audio signal is converted to digital which can cause out of phase artifacts.

Wireless Microphone Systems

Wireless Microphone Systems consist of many receivers in one rack mountable box or 'mainframe' (usually mains powered), the receivers share the two Diversity aerials which are amplified first (to avoid having to rig two aerials per receiver) and the condition of all the transmitters can be seen at a glance, such as battery status and signal strength plus you can listen to the output of each radio microphone without having to plug your headphones into each receiver in turn. Where several mainframes are daisy chained together this is even more useful.

These systems are usually used for large touring companies e.g. Musicals, Theatre and in Television studios.

Wireless In-Ear-Monitoring (IEM)

As briefly mentioned above, a variant on the wireless microphone system is the Wireless In-Ear-Monitor system or IEM, this is essentially a wireless microphone system in reverse. The signal that the performer(s) needs to hear is fed from the audio mixer to the transmitter and then transmitted to a body pack receiver that the performer is wearing, this has a volume control to set the level of the attached ear piece (moulded ear piece or headphone earpiece). Yet again this can be a single IEM package, a slightly more complicated package with one transmitter and two or more receivers set to the same frequency, where each performer needs to hear the same signal, or a multi-channel (multi-frequency) mainframe system that allows each performer to hear just what they need to hear.

Summary

Wireless microphones allow freedom of movement and for the most part can be used without any concerns about quality, though the compansion circuit can cause problems with some sound sources e.g. accordian.

Radio interference is the main concern, where possible use the UHF diversity systems for best audio quality and reduced interference. Receivers are usually mains powered so film and video sound recordists will need to buy or rent a small battery powered receiver system. Per microphone they are still expensive but their cost has dropped sufficiently to the point where if just a few are needed their cost is not prohibitive. If you need more than one or two then renting is the answer.